A major issue facing the modern cellular systems is inter-cell interference which is caused by neighboring cells transmitting at the same time and frequency slots. This eventually leads to severe performance degradation and might even cause connection loss. The resource assignment issue is relevant to all modulation technologies, from the channel assignment in first analog systems to the subchannel assignment in the most recent Orthogonal Frequency Division Multiple Access (OFDMA) which became the underlying transmission technology for 802.16e (WiMAX) and Long Term Evolution (LTE).
There are several approaches to reduce the influence of inter-cell interference. The most common approach is to employ a frequency reuse pattern and by that, avoiding usage of the same frequency bands at adjacent cells. Still, the drawback of this approach is that only a fraction of the frequency resources may be used in each cell, while preferably one would like to reuse the whole available frequency spectrum within every cell.
In CDMA systems the “reuse 1” approach has been adopted. In these systems, the same resources are used in all cells. As a result, the C/I (carrier-to-interference) ratios at cell edges reach low values, in the order of −5 dB. These low C/I values are treated in CDMA systems by a combination of partial loading (fraction of the spreading codes used) and of soft handoff (use of same resource in more than one cell).
Another approach to improve the spectral efficiency in cellular systems is the “reuse partitioning” approach, as described, for example, in S. W. Halpern, “Reuse partitioning in cellular systems,” 33rd IEEE VTC, pp. 322-327, May 1983. Further review can be found in I. Katzela and M. Naghshineh, “Channel assignment schemes for cellular mobile telecommunication systems: A comprehensive survey,” IEEE Personal Communications, vol. 3, pp. 10-31, June 1996. The “reuse partitioning” method divides the frequency resource into two parts or more. First part is used for edge of cell regions, while second part is used for the regions closer to the base station. The first part is used with a conventionally designed reuse factor, appropriate for the cell edges. The second part (covering the inner part of the cell), however, can be used with a higher reuse factor because the Signal to Interference and Noise Ratio (SINR) is higher in this part of the cell in view of stronger desired signal and larger distance from the interferers. An example of such approach, for example is to divide the available channels into 4 channels, three of which are used in a reuse-3 pattern for covering the cell edge regions, while the fourth channel is used in a reuse-1 manner for the inner regions of the cells.
A better understanding of Fractional Frequency Reuse (FFR) is required in order to fully comprehend the various aspects of present invention. Frequency reuse 1 is achieved when all the cells within a network are operative at the same frequency channel. However, frequency reuse 1 in a cellular network implies that users at the cell edge (which constitutes a significant fraction of cell's area) may suffer from substantial inter-cell interference due to transmissions from adjacent cells. For example, if a subscriber is located at the same distance from base stations A, B and C, and the subscriber listens to base station A, the signal of base stations B and C would be regarded as unwanted interference. Thus, from the point of view of the subscriber, it is desirable that the resource (such as frequency channel) over which the subscriber listens to base station A will not be used in base stations B and C. One solution for the inter-cell interference is using reuse-N patterns in which one resource is used in each cell/sector and the resource is reused every N (e.g. 3) cells. Examples of frequency reuse 3 are shown in FIGS. 1A and 1B. On one hand, frequency reuse 3 systems achieve acceptable interference conditions at the cell border, but on the other hand the resource utilization in only third of its full potential.
The “reuse partitioning” approach is exemplified in FIG. 2A. Resources 1, 2 and 3 are used at the cell edges in a reuse-3 pattern, while resource 4 is used in a reuse-1 pattern at the inner part of the cell. In this example, two out of four resources are used in each cell.
Another method to improve upon the regular reuse-3 deployment is by implementing power-based Fractional Frequency Reuse (FFR), where users at the cell center are allowed to operate at all available resources but at lower power, while users located at the cell edge are allowed to operate only at a fraction of the resources available at the cell. A cell center is defined as the area closer to the respective base station that is practically immune to inter-cell interference. An illustration of this option is shown in FIG. 2B. This fractional resources' utilization enables subscribers located at adjacent cells' edges to operate while using different resources, thereby diminishing the inter-cell interference. An example of this option is a mix of reuse-3 for serving edge-of-cell users, where the expected inter-cell interference is strong, and reuse-1 for serving the users located at the inner part of the cell. Fractional frequency reuse schemes provide more uniform distribution of C/I over the cell and provide higher aggregate throughput.
The description of reuse schemes above is focused on omnidirectional cells, each having a base station installed at its center. The above considerations, however, apply also to sectored base stations. The most common arrangement in cellular industry is using 3-sector base stations. Sectors of a base station are sometimes referred to in the literature as “edge-illuminated cells”. In this case, the interference between adjacent sectors is controlled by the patterns of the antennas used to illuminate the sectors.
In the Applicant's co-pending US application published under No. 20100075687, the transmission resources are allocated for conveying communications via each of a plurality of beam generating means so that the transmission resources allocated to each of the beam generating means is different from the transmission resources allocated for conveying communications via any of angularly adjacent beam generating means at the respective base station, and are different from the transmission resources allocated for conveying communications via beam generating means associated with adjacent base stations and directed towards geographical areas located in a proximity to the geographical area towards which the respective narrow beam is directed.
Still, it is required to implement a reuse scheme in which larger fraction of the resources can be used at the edge of the cell—½ or ⅔, as opposed to ⅓ in the typical FFR scheme, thereby enhancing the cell throughput.